The invention generally relates to exhaust emission treatment for Diesel engines. More specifically, the invention concerns a method and apparatus for regeneration of a nitrous oxide (NOx) adsorbent placed in the exhaust stream of a Diesel engine.
It is well known that Diesel engines emit significant quantities of NOx and that these engines produce exhaust that is characteristically lean—i.e. the exhaust contains significant quantities of oxygen. These combined conditions make acceptable exhaust after-treatment of NOx hard to achieve, in that conducting chemical reduction in an oxidizing atmosphere is difficult. Perhaps the most promising approach to meeting the NOx emissions objective is by means of an NOx adsorber which must be periodically regenerated with chemical reductants. The traditional reductant of choice has been the Diesel fuel itself which, when injected into the exhaust stream upstream of the NOx adsorber is, ideally, thermally modified to hydrogen and carbon monoxide, which are the active reducing chemical species. See for example, U.S. Pat. No. 5,406,790 to Hirota, et al. It is also well known that hydrogen is the most effective reducing agent, followed by mixtures of hydrogen and carbon monoxide, followed by carbon monoxide alone, and followed by light hydrocarbon species. For a combination of reasons, this approach has not been commercially successful.
NOx adsorber performance is highly contingent upon very low sulphur exposure and upon the efficiency of the regeneration process. Regeneration of the adsorber is the subject of this invention.
Diesel fuel has conventionally been used as the reductant by introducing fuel to the exhaust stream at one of two locations. The first location is at the engine where the same injection equipment used for fueling the engine is used at a different time—the post combustion regime—to introduce the fuel to the exhaust stream. Injecting in this manner leads to problems of oil dilution, particulate formation and a molar hydrogen-to-carbon monoxide ratio derived from the Diesel fuel of less than one.
The second conventional fuel injection site is down-stream of the engine and up-stream of the NOx adsorber. Injection at this location leads to problems of particulate formation, deposit formation in the injector nozzle, and a low molar hydrogen-carbon monoxide ratio.
In addition, both of these methods produce a hydrocarbon emission which must be resolved with a downstream oxidation catalyst. Also, because of the so-called hydrocarbon slip, there is an excessive fuel economy penalty in the range of 3-7% associated with this approach of using the fuel itself as the reductant source.
Therefore, there is seen to be a need in the prior art for a method and arrangement for supplying a reductant to an exhaust stream, for example to effect NOx adsorber regeneration with a minimum of required new fueling system infrastructure and with no appreciable impact on fuel economy.